Sixth seed Stefanos Tsitsipas won his opening match at the ATP Finals by edging out fourth seed and fellow debutant Daniil Medvedev in London.
The Greek 21-year-old beat the Russian 7-6 (7-5) 6-4 at the O2 Arena.
There was just one break of serve in the match with Tsitsipas, the youngest player in the tournament, earning his first win over Medvedev.
World number one Rafael Nadal plays defending champion Alexander Zverev later on Monday in the same group.
Novak Djokovic, who can overtake Nadal at the top of the world rankings this week, leads the other group after winning on Sunday as the top eight players of the year compete in the season-ending event.
The top two players in each group after the round-robin stage progress to the semi-finals on Saturday.
Read the full report here.